1. Field of the Invention
This invention relates to an image signal restoring apparatus for restoring a digitized image signal.
2. Description of the Related Art
The image signal restoring apparatuses of the kind arranged to restore a digitized image signal include, for example, an apparatus arranged as shown in FIG. 1 of the accompanying drawings.
FIG. 1 shows in outline the arrangement of the conventional image signal reproducing apparatus for reproducing a digital image signal from a recording medium on which digitized image signals are recorded. Referring to FIG. 1, a reproduced signal obtained from a recording medium which is, for example, a video tape, is applied from an input terminal 1 to a data reproducing circuit 2. The data reproducing circuit 2 detects a synchronizing code included in the reproduced signal and reproduces all data in accordance with a clock signal synchronized with the synchronizing code. The data reproduced by the data reproducing circuit 2 is applied to an error correcting circuit (hereinafter referred to as ECC) 3. The ECC 3 corrects any code error in a known manner. After error correction, the data is supplied to a data separation circuit 4. The data separation circuit 4 separates address data which is included in the reproduced data to supply the address data to a memory control circuit 6. Meanwhile, image data included in the reproduced data is supplied to a frame memory 5.
The memory control circuit 6 causes image data to be written into the frame memory 5 in accordance with the address data separated. An error flag which corresponds to each image data is supplied from the ECC 3 to the memory control circuit 6. The error flag is supplied for the purpose of preventing any image data that cannot be corrected by the ECC 3 from being written into the frame memory 5. In other words, the memory control circuit 6 is arranged to generate writing addresses according to the address data received from the data separation circuit 4 and to generate a writing-enable signal according to the error flag supplied from ECC 4.
Reading from the frame memory 5 is performed in a predetermined order in a frame cycle. As a result, a reproduced digital image signal is obtained from a terminal 7. The image data obtained for any picture element for which an error has occurred is not written into the frame memory 5. Therefore, the image data which has been obtained for a preceding frame and stored at a frame memory address corresponding to the picture element having the error is read out. Address data are arranged for addresses corresponding to the positions of picture elements on the picture. Therefore, the image data of any picture element at which an error has (incorrigibly) occurred is eventually replaced with the image data previously obtained for a picture element located in the same position on the preceding picture (or frame). In other words, inter-frame interpolation is carried out by the arrangement described.
Meanwhile, a so-called block encoding method has recently been proposed for a high-efficient coding process. The method is based on the fact that picture elements have a high degree of correlativity among them within a picture element block consisting of (n.times.m) number of picture element matrixes, n and m being integers not less than 2. In accordance with this method, the high-efficient coding process is performed on such picture element blocks one by one. One example of the block encoding method has been disclosed in Japanese Laid-Open Patent Application No. SHO 62-151090. In this case, the maximum and minimum values of all the picture elements within each picture element block are obtained and each of the picture elements is quantized into quantized data between the maximum and minimum values. This is a highly efficient encoding method and has a great advantage in that the propagation of any error is limited to the inside of the block in which it occurred.
However, in reproducing the image data encoded by the block encoding method from a recording medium, reproduction by blocks often becomes impossible, because: It becomes impossible to restore all the picture elements within each block when any error takes place in the data of the above-stated maximum and minimum values or in the address data indicating the position of the block on the picture. This inevitably results in a wider unrestorable area on the picture.
Therefore, if the interpolating process is performed with the reproducing apparatus of FIG. 1 in reproducing the block-encoded image data from the recording medium, it is highly possible that the image data for one and the same area on the picture might remain unreproduced over an excessively long period of time. This would deteriorate the picture quality. The picture quality deterioration due to this would more frequently occur particularly in cases where the reproducing operation is to be performed at a speed different from a speed employed in recording.
FIG. 2 shows in outline the arrangement of an image signal restoring apparatus which is arranged to reproduce and restore image data from a recording medium on which the image data is recorded in a state of having been block-encoded in the above-stated manner. Referring to FIG. 2, an input terminal 8 is arranged to receive a reproduced signal including image data which has been encoded by blocks. An error detecting circuit 13 is arranged to detect data errors in accordance with an error detection code included in the reproduced signal received. A decoding circuit 9 is arranged to decode the encoded data coming from the input terminal 8 and to produce, along with the image data, the address of the decoded block of the data, i.e., a block address, which is, for example, a signal indicating the position of the block on the picture. In this instance, data to be decoded by using any data that is detected to have an error by the above-stated error detecting circuit 13 is converted into a given code before it is output. The image data output from the decoding circuit 9 is written into a frame memory 10 in accordance with an applicable block address. A preceding-frame memory 16 is arranged to store image data of a frame preceding a frame for which the image data is stored in the frame memory 10. The data stored in these frame memories 10 and 16 are read out in accordance with reading addresses generated by a reading address generating circuit 17. The stored data is thus output in a predetermined sequence.
An error judging circuit 14 examines the output of the frame memory 10 to find any picture element which has been replaced with the above-stated given code thus indicating occurrence of an error. When the image data of this picture element is supplied to a switch 15, the connecting position of the switch 15 is shifted by the error judging circuit 14 to one of its two sides which is on the output side of the preceding-frame memory 16. The position of the switch 15 is normally on the output side of the frame memory 10. With the switch position shifted as mentioned above, any image data that is omitted due to occurrence of an error is thus replaced with image data located in the same position on the picture of a frame immediately preceding the current frame. The substitute image data is supplied to an output terminal 18. The preceding-frame memory 16 is arranged to have its contents renewed with apposite image data coming from the output side of the switch 15.
Since the dropped-out image data is to be replaced with the corresponding image data of the preceding frame, the image data for the picture is unevenly and variedly renewed. Therefore, in accordance with the above-stated example of conventional arrangement, new image data and old image data are commingled to give an unnatural reproduced image. In the event of a special reproducing operation wherein incoming image data is in an eccentric state, this problem becomes more salient.